In present power supply products, the synchronous rectifier often utilizes a transformer to drive synchronous rectifying transistors for achieving efficient rectifying operation. As shown in the FIG. 1, it is a schematic diagram of a conventional synchronous rectifier 10. In FIG. 1, the synchronous rectifier 10 includes: a input unit 11, a control unit 12 and an output unit 13. Meanwhile, the input unit 11 further includes a signal detecting circuit 111, a rectifying circuit 112, a signal amplified circuit 113, a first transformer T1, a second transformer T2, a third transformer T3 and a bridge rectifying circuit constructed by four transistors, Qa, Qb, Qc and Qd. Furthermore, the output unit 13 includes a first rectifying inductor L1, a second rectifying inductor L2, a rectifying capacitor C, a fourth transformer T4, a first and a second switch control circuit 131, 132, and a third and a fourth switch circuit 133, 134. Of course, in the first switch control circuit 131 further includes a transistor Q1, a first diode D1, a first resistor R1, and a first induction coil L11. And the second switch control circuit 132 further includes a transistor Q2, a second diode D2, a second resistor R2, and a second coil L22. And then the third and the fourth switch control circuits 133, 134 can be a transistor Q3 and Q4. The first transformer T1 further includes a first side coil T11, and a second side coils T12, T13. And the second transformer T2 further includes a second side coil T21, and a second side coils T22, T23. Besides, the third transformer T3 includes a first side coil T31 and a second side coil T32. And the fourth transformer T4 includes a first side coil T41 and a second side coil T42. The theory and the drawbacks of the conventional synchronous rectifier now represent as below.
After an AC input current Iin detected by the detecting circuit 111, then the AC input current Iin is transformed to the second coil T32 through the first side coil T31 of the third transformer T3. Meanwhile, the control unit 12 produces a rectifying control signal In to the input unit 11 to control the conducting sequences of the transistor Qa, Qb, Qc and Qd, and to proceed the power transmission. The rectifying control signal In is amplified by the signal amplifying circuit 113 and then have a control signal Iac and Ibd.
Because the control signal Iac is transformed to the second coil T12 and T13 through the first coil T11 of the first transformer T1, the gate electrode of the transistors Qa and Qc, which are electrically connected to the second side coil T12 and T13, can generate a gate voltage Vag and a gate voltage Vcg. Therefore, the control signal Iac can control the transistors Qa and Qc to be in a turn on state or a turn off state. By the same reason, the control signal Ibd can make the transistor Qb and Qd to generate a gate voltage Vbg and a gate voltage Vdg on the gate electrodes thereof through the second transformer T2. And make the transistor Qb and Qd to be in a turn on state or a turn off state. Therefore, by the bridge switch circuit consisted of the four transistors Qa, Qb, Qc and Qd, the direct input current Vin can be transformed to the output unit 13 through the fourth transformer T4.
Furthermore, the current signal Ip1 and the voltage signal Vp1 are inputted into the fourth transformer T4 of the output unit 13, and then be transformed by the first side coil T41 of the fourth transformer T4, and then the second side coil T42 produces another one current signal Ip2 and voltage signal Vp2. The first and second switch control circuits 131, 132 are electrically connect to the second side coil T42. So that the first induction coil L11 of the first switch control circuit 131 can produce a current Ip21 according to the current signal Ip2. Besides, the transistor Q1's gate electrode forms a gate voltage V1g to control the transistor Q1 in the turn on or the turn off state. Of course, according to the current signal Ip2, the second induction coil L22 of the second switch control circuit 132 can produce a induction current Ip22. And the transistor Q2's gate electrode forms a gate voltage V2g to control the transistor Q2 to be turned on or turned off.
So, depends on the switching between the turned-on states and the turned-off states of the transistors Q1 and Q2, and co-operates with the first and second rectifying inductors L1 and L2, the rectifying action of the rectifying capacitor C can transform the current signal Ip2 into a rectifying output signal for outputting itself. Certainly, the rectifying output signal includes a rectifying output current lout and a rectifying output voltage Vout.
According to the above explanation, the conventional synchronous rectifier's 10 control scheme, which is for controlling the conduction state of the transistors Q1 and Q2, is mainly depended on the induction currents Ip21 and Ip2 produced by the first and second induction coils L11, L23, and depended on the gate voltages V1g, V2g formed in the transistor Q1, Q2, to drive the transistors Q1 and Q2. However, by the restriction of the leaking induction phenomenon, the conventional synchronous rectifier 10 is can't constructs an accurate driving control signal, e.g. the gate voltages V1g, V2g. Therefore, the goodwill for raising the efficiency of rectifying is not so well as the prediction of the conventional synchronous rectifier 10.
Furthermore, please refer to the FIG. 2(a), which is the wave form drawing of the driving control signal of the conventional synchronous rectifier 10 for controlling the first and second switch control circuits 131, 132. The FIG. 2(a) includes the wave forms of the gate voltages V1g, V2g for driving the transistors Q1, Q2, and the wave form of the voltage signal Vp2, which is transformed form the voltage signal Vp1 by the first side coil T41 of the fourth transformer T4, and then form in the second side coil T42. It is obviously that the gate voltages V1g, V2g can change its level follow the exchanges between the high level H and the low level L of the voltage signal Vp2. Furthermore, FIG. 2(a) shows the transient response of the gate voltages V1g, V2g in different times t1˜t8. But, according to the FE1 which represents the falling edge waveform of the gate voltage V1g in time t5 and the FE2 which represents the falling edge waveform of the gate voltage V2g in time t7, it can be understood that while the gate voltage V1g is from a high level H to a low level L, the gate voltage V2g varies at the same time. However, because of the leaking induction phenomenon, the gate voltage V2g should passes a raising period that it can completes a transformation from a low level L to a high level H. By the same reason, while the gate voltage V2g is from a high level H to a low level L, the gate voltage V1g should passes a raising period that it can completes a transformation from a low level L to a high level H. Therefore, the transistors Q1, Q2 is controlled by the gate voltages V1g, V2g so that the cross conduction phenomenon is hard to overcome.
And in the FIG. 1, the functions of the third, fourth switch control circuits 133, 134, which are individually connect to the gate electrodes of transistors Q1, Q2, should cooperate with the FIG. 2(b) which shows the conventional synchronous rectifier 10 electrically connected to another synchronous rectifier 50. In the FIG. 2(b) the conventional synchronous rectifier 10 is parallelly connected to another power supply 5. It is often to provide a output current detector S connected to a common output terminal of the power supplies 1 and 5, for preventing a reverse current Ir, which is produced by the input unit 51 of the synchronous rectifier 50, passing the aforesaid parallel connection to destroy the power supply 1 or make it malfunction. While the output current detector S detects the reverse current Ir, the output current detector S creates a sensing signal Is to the control unit 12 of the synchronous rectifier 10, and then the control unit 12 produces a voltage signals V3g and V4g, and separately sends the voltage signal V3g and V4g to the gate electrode of the transistors Q3 and Q4. So the transistors Q1 and Q2 can be forced to cut off the connection with the transistors Q3 and Q4 for preventing the power supply 1 from being destroyed by the reverse current Ir.
However, the weakness of the prior art is that although the output current detector S connect with the output terminal of the synchronous rectifier 10 can solve the problem from the reverse current Ir, but the electric power loss will increase when the electric load getting high. So, the prior art cannot really effectiveness to increase the transportation efficiency of the electric power.